U.S. Pat. No. 4,504,727 sets forth a laser drilling system which utilizes photoacoustic feedback to monitor and control the laser beam so as to control a laser drilling process. In the -727 patent, the drilling process is provided to produce an array of holes in a multilayered printed circuit board. In such arrangements, it is necessary to closely control the pulse power of a laser so that it can be adjusted for each successive layer in such structures. There is no suggestion in the -727 patent that the laser formed hole be later precision finished to a close tolerance suitable for use in precision small diameter holes in nozzle parts for fuel injection systems or the like.
U.S. Pat. No. 3,696,504, issued Oct. 10, 1972, discloses a process wherein a laser beam is used to form a hole through a part. In the case of metal parts, it is recognized that such laser formed holes can effect the metallurgy of the parts. In accordance with the process of the -504 patent, the altered metallurgy of the laser formed hole is subsequently machined away by a mechanical tool having a blade which reams the wall of the laser formed hole.
When drilling precision small diameter holes for diesel engine fuel system nozzles, it is critical that the spray holes formed in the nozzle have precise diameters and a smooth internal microfinish to produce desired flow rates and angle accuracy of fuel discharge into engine combustion chambers.
In the present invention, undersized holes are first formed by a laser to reduce the cycle time for penetration of the hole through the part. The part is then finished by electrical discharge machining. Specifically, a wire electrode of an electrical discharge machine is advanced through the previously laser formed undersized hole which is sized to provide an annular flow path for unidirectional flow of electrolyte during an EDM process. The flow path is sized so that particles produced during the EDM process will be flushed from the part by the unidirectional flow of electrolyte. The wire electrode is sized and is connected to a pulse generator to control energy at a spark gap between the wire electrode and a grounded workpiece so as to form a hole of precision dimension and with a microfinished surface thereon. The use of a laser to form a rough undersized hole, followed by a precision finishing of the laser hole by electrically discharge machining the inner surface thereof reduces the total hole forming cycle time as compared to the cycle time of prior computer controlled EDM hole drilling equipment. Furthermore, the combination of laser drilling of undersized holes and EDM finishing enables nozzle parts to be produced at high production rates while maintaining hole size precision and finish.